1. Field of Invention
The present invention relates to a thin film transistor (TFT) and method of fabricating the same and more particularly, to a TFT and its fabricating method having a Metal Induced Crystallization (MIC) region crystallized by an MIC process and a Metal Induced Lateral Crystallization (MILC) region crystallized by an MILC process wherein the boundary between the MIC and MILC regions is located outside a channel region of the TFT.
2. Discussion of Related Art
A method of crystallizing amorphous silicon using heat treatment at a low temperature after a certain kind of a metal layer has been deposited on the amorphous silicon is known as an MIC process. The MIC process is beneficial due to the low temperature crystallization of amorphous silicon. However, the MIC process has not been applied to electronic devices because of an inflow of metal into the thin film of crystallized silicon formed underneath the metal layer, which causes the intrinsic characteristics of amorphous silicon to deteriorate.
A different method of crystallizing amorphous silicon by MILC has been proposed to address this problem. FIG. 1A to FIG. 1D show examples of schematic cross-sectional views for explaining an MILC process.
Referring to FIG. 1A, an amorphous silicon layer 11 to be crystallized is formed to a thickness of 1000 .ANG. on an insulating layer (not shown). Metal patterns, such as nickel patterns 13, are formed on the amorphous silicon layer 11.
Referring to FIG. 1B, the amorphous silicon layer 11 is crystallized when heat is applied to the nickel patterns 13 at a temperature of 350.degree. C.-500.degree. C. On the regions "A" of the amorphous silicon layer 11 having the nickel patterns 13 thereon, nickel silicide 14 is formed as the nickel in the nickel patterns 13 reacts with the amorphous silicon in the layer 11. The nickel silicide 14 becomes a seed for crystallization and promotes the crystallization of the amorphous silicon layer 11. The "A" regions crystallized directly by the nickel silicide 14 become the MIC regions.
Referring to FIG. 1C, the boundaries of the "A" regions having crystallized silicon function as a new seed for crystallization and cause lateral crystallization of silicon in the region "B". Since the region "B" has no seed of crystallization and has not been solidified yet, the lateral crystallization of silicon is performed by the MIC regions which have been completely crystallized. That is, the region "B" becomes the MILC region as the crystallization by the nickel silicide 14 is induced in the lateral direction of the MIC region.
FIG. 1D shows a cross-sectional view of a crystallized silicon layer having MIC and MILC regions. Generally, the MILC region have less metal contamination, superior crystals and less coarseness in the crystallized surface thereof, than the MIC regions. Thus, the MILC regions are more suitable to function as channel regions for TFTs.
FIGS. 2A to 2C show a method of fabricating a channel region of a thin film transistor using an MILC process according to a related art.
Referring to FIG. 2A, an amorphous silicon layer as an active layer 21 is deposited on an insulation substrate 20 having a buffer film (not shown) on its upper part, and the active layer 21 is patterned by photolithography and etching. A gate insulation layer 22 and a gate electrode 23 are formed on the active layer 21 by conventional processes.
Referring to FIG. 2B, a nickel layer 24 is formed to a thickness of 20 .ANG. by sputtering nickel on the entire surface of the formed structure. Then a source region 21S and a drain 21D are formed at portions of the active layer 21 by heavily doping the entire surface of the formed structure with impurities. Between the source and drain regions 21S and 21D, a channel region 21C is formed on the substrate 20.
Referring to FIG. 2C, amorphous silicon in the active layer 21 is crystallized by heating the substrate 20 at a temperature of 350.degree. C.-500.degree. C. Then the source and drain regions 21S and 21D on which the nickel layer 24 has been formed become the MIC regions having silicon crystallized by an MIC process. The channel region 21C without the nickel layer 24 formed directly thereon, becomes the MILC region where silicon has been crystallized by an MILC process. Impurities are activated in the source and drain regions 21S and 21D during the heat treatment as amorphous silicon is crystallized in the active layer 21.
In the thin film transistor fabricated by the above-described method according to the conventional art, the channel region 21C has boundaries defined by the crystalline structure of silicon in the MIC regions facing that of silicon in the adjacent MILC region. Since the boundary between the MIC region and the MILC region is located at the junction where the source or drain region meets the channel region, an abrupt difference in the crystal structure appears in the junction and the metal from the MIC region contaminates the adjacent MILC region. Consequently, a trap is formed at such junctions as soon as the TFT is turned on which causes unstable channel regions and deteriorates the characteristics of the thin film transistor.